The present invention relates to a display apparatus capable of displaying mixed alphanumeric and graphic images.
The earliest display terminals used as computer data input/output terminals were alphanumeric displays. Subsequently graphic display terminals came into use, initially mainly for military and space purposes but latterly increasingly for commercial and industrial purposes. Typical alphanumeric displays are the IBM 3277, 3278, 3279 and 8775 display stations. Typical graphics displays are the IBM 2250, 3250 and 5080 displays: the IBM 2250 and 3250 displays are directed-beam cathode ray tube (CRT) displays while the IBM 5080 display employs a color raster-scanned CRT. Cathode ray tubes remain the principal display device for many reasons including cost, versatility, efficiency and relative ease of providing color. Although CRTs require refresh, this is not now regarded as a serious drawback in view of the decreasing cost of semiconductor memory (and increasing memory speeds).
At the present time, most alphanumeric displays employ coded character buffers, either mapped or unmapped. Character codes in a mapped buffer correspond to character positions in the screen, whereas an unmapped buffer requires each character code to have its screen address stored therewith. The character codes stored in the refresh buffer serve as pointers to a character generator which contains the bit patterns required to display that character on the CRT screen. Associated with the character codes are attribute codes which determine how the character is to be displayed on the screen. Coded character buffers have proved very successful for alphanumeric displays in view of their relatively small memory requirements. If a loadable character generator is used, graphic pictures can be built up by employing special character shapes or symbols. Although in theory, exactly the same graphics image could be displayed with a coded buffer alphanumeric display as with a full graphics display, in practice there is a limit, partly set by economic considerations and partly by performance considerations. To display a complicated and high-resolution graphics image in an alphanumeric coded-character display would require a very large random access memory in the character generator and too much time loading the required bit patterns into the character generator.
U.S. Pat. No. 4,217,577 describes a character graphics color display system in which graphical images are built up from image or character cells. The specification discusses the various factors which need to be considered when mixing lines of different colors but generally describes a basically alphanumeric display with additional character graphic capability allowing quite complex charts to be displayed. However, for the reasons mentioned above, the described apparatus is not entirely satisfactory for high-function, high resolution, highly-interactive graphics applications.
Bit-for-pel or all point addressable (APA) refresh buffers are fast becoming the standard requirement for fully-interactive high-resolution graphics displays. In such a display, each addressable picture element (pixel or pel) on the screen is represented by one or more bits in the refresh buffer--color displays require 3 planes of memory, red, blue and green. Although such graphics displays are excellent for displaying graphics images and can display any size or shape of alphanumeric character, the latter is quite inefficient in an APA buffer compared with a coded character refresh buffer. For example assume that each character is built up from a matrix of 20.times.9 pels. In an APA buffer this requires 20.times.9=180 bits. For a color display this means at least 540 (180.times.3) bits are required for each character. In contrast a coded buffer requires only one byte to specify the character and one attribute byte (for color etc.), that is, 16 bits. To change one character on the screen, only these 16 bits need to be changed (and perhaps only the 8 bits specifiying the character if the attributes are unchanged) in contrast to the 540 bits in the APA buffer. Thus from this point of view, the coded buffer can be up to thirty times faster than an APA buffer for changing a character on a color display. Some machines, such as the IBM Personal Computer, allow switching between character and APA modes.
The published PCT patent application No. 83/02509 describes a raster-scan CRT display apparatus in which separate all points addressable memories are used for alphanumeric and graphic images. The alphanumeric memory is effectively an extra plane of APA memory and contain priority bits for determining whether the graphics or alphanumeric data are foreground or background. Alphanumeric-representing bits are read out 8 "pels" at a time with a further six bits representing their color and a further two priority bits controlling the eight alphanumeric bits. The disadvantages with this arrangement are that extra memory is required for the alphanumeric characters than would be required if a coded character buffer were used and increased writing time is required for changing alphanumeric characters. Chromatic Inc's CGC 7900 display system is also an APA graphics display in which two images can be overlayed with different foreground or background colours.
European Patent Specifications EP -A-0096627 and EP -A-0108516 disclose mixing arrangements in which a particular color code (normally black) is used to represent a color signal which enables text data to be displayed instead of graphics data on a pel-by-pel basis. The code represents a transparent color and requires a code for each pel. The article "A VLSI Controller for Bit-Mapped Graphics Display" by James Kahn in Wescon Technical Papers, Vol. 26, September 1982, pages 33/2-1 to 33/2-13 discloses an arrangement in which graphics and alphanumeric data are mixed in a multiplexor. However no details of the control of the multiplexor are given.